Many manufacturing processes including those for the production of nitric acid, sodium nitrate, or hydroxylamine disulfonate diammonium salt result in emissions which may include NO.sub.x (where x=1 or 2), SO.sub.2, and mist. Kirk-Othmer, "Encyclopedia of Chemical Technology", Volume 17, pages 93-94 (1996) teaches processes for making nitric acid and three methods for abating NO.sub.x emissions in nitric acid production to an acceptable level for discharge to the atmosphere. The three methods include extended absorption (reaction of nitrogen oxides with water to form acid), selective catalytic abatement (catalyst and ammonia fuel selectively reduce nitrogen oxides in preference to combustion), and nonselective abatement (catalyst and propane or natural gas reduce nitrogen oxides to nitrogen).
Kirk-Othmer, "Encyclopedia of Chemical Technology", Volume 22, pages 383-393 (1997) teaches processes for making sodium nitrate.
Hydroxylamine is widely useful in the transformation of organic compounds to derivatives, which in turn may be intermediates in pharmaceutical or industrial synthesis of other complex molecules. An important use of hydroxylamine is captively in the synthesis of caprolactam which may then be polymerized to polycaprolactam.
The classical method for the production of hydroxylamine is that ascribed to Raschig. An important commercial process consists of the reduction of ammonium nitrite with ammonium bisulfite and sulfur dioxide to give hydroxylamine disulfonate diammonium salt. Upon hydrolysis, hydroxylamine disulfonate diammonium salt gives hydroxylamine sulfate, an intermediate in the production of caprolactam. The overall reaction for production of hydroxylamine disulfonate diammonium salt is
(1) NH.sub.4 NO.sub.2 +NH.sub.4 HSO.sub.3 +SO.sub.2 .fwdarw.HO--N--(SO.sub.3 NH.sub.4).sub.2 where PA1 (2) NH.sub.4 OH+SO.sub.2 .fwdarw.NH.sub.4 HSO.sub.3 PA1 (a) from the manufacturing stream, feeding the emissions comprising NO.sub.x to a scrubbing reactor; PA1 (b) to the emissions in the scrubbing reactor, adding alkali solution and NO.sub.2 in an amount sufficient so that a substantial amount of the NO.sub.x forms ammonium nitrite or ammonium nitrate while maintaining the pH of the formed ammonium nitrite solution at greater than about 7.5 and vent gas comprising mist exits from the scrubbing reactor; PA1 (c) maintaining the temperature of the vent gas at less than about 40.degree. C.; and PA1 (d) removing the mist from the vent gas. PA1 (a) from the manufacturing stream, feeding the emissions to a scrubbing reactor; and PA1 (b) to the emissions in the scrubbing reactor, adding alkali solution and NO.sub.2 in an amount sufficient so that a substantial amount of the NO.sub.x forms ammonium nitrite or ammonium nitrate and a substantial amount of said SO.sub.2 forms ammonium sulfite or ammonium sulfate while maintaining the pH of the formed ammonium nitrite solution at greater than about 7.5 and vent gas comprising mist exits from the scrubbing reactor; and PA1 (c) maintaining the temperature of the vent gas at less than about 40.degree. C. Preferably, step (d) is removing mist from the vent gas.
In commercial practice, the reaction is conducted in a packed tower or column where a solution of the reactants is recirculated, an aqueous nitrite solution is fed to the recirculating reactant stream, and supplemental ammonia, such as ammonium hydroxide or ammonium carbonate, is added in an amount to satisfy the stoichiometric requirements NO.sub.2.sup.- :NH.sub.4.sup.+ =1:2 and SO.sub.2 .uparw., obtained for example by the combustion of sulfur with air, is fed at the bottom of the packed column and is vigorously contacted with the liquid in the packed section of the tower. The amount of SO.sub.2 fed to the tower should also satisfy the requirements of equation (1) above and the three reactants should therefore be fed in the molar proportions NO.sub.2.sup.- :NH.sub.4.sup.+ SO.sub.2 =1:2:2.
The reaction is complex, with the possibility of many side reactions which have been discussed authoritatively in a review by F. Seel, Fortschr. Chem. Forsch. 4, 301-332 (1963). Side reactions do occur and as a result, NO.sub.x gases, predominantly NO, are released. When the reaction is carried out at lower temperatures, yields of hydroxylamine disulfonate diammonium salt are improved and the extent to which side reactions occur and NO.sub.x are formed is decreased. But even at 0.degree. C., a temperature which is expensive to maintain, NO.sub.x emissions, primarily NO, still constitute up to 3,000 volume ppm in the vent gas.
A known process for abating NO.sub.x emissions is taught by commonly assigned U.S. Pat. No. 4,996,036. The process treats emission gases from the production of hydroxylamine disulfonate diammonium salt and comprises adding NO.sub.2 to the emission gases and contacting the mixture of NO.sub.2 and emission gases with an aqueous alkali solution sufficiently to form nitrite and nitrate ions in the alkali solution. Unfortunately, this patent illustrates a lab scale process which was operated for only four hours and not in a continuous manner. When we tried to scale up the process taught by the patent and then operate a continuous process, the result was unsatisfactory.
Thus, a need exists in the industry for an improved emission abatement process which is of commercial scale and operated continuously.